Validates method should not change options argument

when validating a record, if a validation context is used, use the same context when validating related records 

use thread variable in TaggedLogging

Wrong behavior of ActiveModel::Errors#dup is causing regressions on Rails master

Since ActiveModel::Errors instance keeps all error messages as hash
we should duplicate this object as well.

Previously ActiveModel::Errors was a subclass of ActiveSupport::OrderedHash,
which results in different behavior on `dup`, this may result in regression for
people relying on it.

E.G.:

```ruby
class Parent < ActiveRecord::Base
  has_one :child
  validates_presence_of :name, :on => "custom_context"
  validates_associated :child
end

class Child < ActiveRecord::Base
  belongs_to :parent
  validates_presence_of :name, :on => "custom_context"
end

p = Parent.new(:name => "Montoto", :child => Child.new)
p.valid?(:custom_context) # => Returns true, even though the child is not valid under the same context.
```

previous solution can cause race conditions under GIL-free ruby implementations

See the CHANGELOG for details.

Fixes #950.

This reverts commit c99d507f.

Stub ip method of alternative request class in router tests